The usual sight for military and similar rifles consists of a small solid cylinder mounted above the barrel near its tip and a small metal block with a vee-shaped notch also mounted above the barrel near its rear end. The axis of the cylinder is parallel to the barrel and appears to just touch the sides of the vee-notch when the users line of sight is boresighted with the weapon. The metal block or rear sight is adjusted vertically to compensate for range variations and horizontal for wind effects.
This system has a number of drawbacks. First, the positions of the front and rear sights make it impossible for the user to focus on both sights and the target at the same time. Telescopic sights are available which solve the focussing problem, but these are expensive and limit the user in quickly locating targets of opportunity due to their narrow field of view. The problem becomes even more complicated when the user attempts to use these systems while wearing a pair of night vision goggles or binoculars, which impose additional focusing problems.